Although speed/torque may be a minor factor I beleive the primary reason for 400cps (cycles per second, used when this system was designed, now known as Hertz) is the saving in iron needed in magnetic functions such as transformers and motors.
The reason iron is reduced is that the inductance required goes down as frequency increases.
The reason that 400hz was chosen is that with available materials avalable at the time hystersis and other losses become unexcetable at higher frequencies.
To contrast some modern switching power supplies operate at well over 1Mhz and use tiny inductors.
Last edited by MurphyWasRight; 1st June 2011 at 19:30.
Reason: Added note on motors, added missing 'at' in at the time